Since it's a login form, as I said, the answer is maybe yes, maybe no.
Examine the form. See what fields it asks for in the fields and check to see the name of the page it's posting to. Let's hope it's easy like two fields (maybe called user and password). If so, then open an iframe or a popup window with a link to the form's target page. The url of the page it should open is:
http:// www . site#2 . com / target.php ? user = XXXXX & password = YYYYYY
By doing this, you will have logged in the user and the cookies will be set on their computer. Once that's done, now you can show them deep links (targetting a new window) directly to the content you desire and because they have the right cookies, they will go to that page (and they will be on that page).
By the way, the term for what you're trying to accomplish is single sign on. That is, they exist on your web site but they automatically get logged into another web site without having to enter their username and password on the other site. It's useful when two companies merge and they have separate web sites but their customers should be able to move about freely from site to site.
The technique I just showed you works about 90% of the time. Sometimes, the sites use random location redirects to prevent the technique I'm showing you. In that case, you sometimes need to open the target window and then use Javascript from the source window to trigger actions in the target window. And once, I found that site #2 had that prevented too. (That company charged $1000 for having the privledge of using single sign on so they blocked automated techniques unless you pay them to turn off the blocks for your account). But, as I said, that's only happened to me once. Usually, the iframe, popup, or javascript techniques will work.
